1. Field of the Invention
This invention relates to an apparatus for suppression of spurious responses in crystal filters. More particularly, this invention relates to an apparatus for suppressing undesired attenuation poles in the frequency response characteristic curves for pedestal mounted crystal filters.
2. Background of the Invention
A known pedestal mounting arrangement for supporting a crystal 10 is shown in FIG. 1. A substantially similar crystal mounting arrangement is described and claimed in U.S. Pat. No. 4,282,454, entitled "Piezoelectric Crystal Mounting and Connection Arrangement", issued to Wakat, Jr. et al. on Aug. 4, 1981 and having the same assignee as the present invention. The contents of U.S. Pat. No. 4,282,454 are hereby incorporated by reference. U.S. Pat. No. 4,334,343 for a "Method of Making Crystal mounting and Connection Arrangement" issued to Wakat, Jr. et al. on June 15, 1982 also shares a common assignee with the present invention. The contents of U.S. Pat. No. 4,334,343 are hereby incorporated by reference. A U.S. Pat. application Ser. No. 408,409 to Charles Shanley, filed Aug. 16, 1982, now U.S. Pat. No. 4,430,596 issued Feb. 7, 1984, discloses and claims an improved pedestal mounting arrangement and is entitled "Temperature Insensitive Piezoelectric Crystal Mounting Arrangement". The contents of such U.S. Pat. No. 4,430,596 are hereby incorporated by reference.
Referring now to the cross-sectional view of FIG. 1 viewed in conjunction with FIG. 2A and FIG. 2B, crystal 10, which may be a polished AT cut quartz blank, includes opposed major surfaces, namely upper surface 15 and lower surface 20. Disposed on upper surface 15 is a plurality of electrically conductive upper surface electrodes 25, 30, 35 and 40. Located substantially opposite these upper surface electrodes are lower surface electrodes 45, 50, 55 and 60 respectively. These upper and lower surface electrodes may, for example, be made of aluminum and are situated in such a manner to form a crystal filter network wherein electrode pair 25 and 45 form a first resonator, electrode pair 30 and 50 form a second resonator, electrode pair 35 and 55 form a third resonator, and electrode pair 40 and 60 form a fourth resonator. In this arrangement, the first and second resonator are substantially adjacent and acoustically coupled together. Similarly, the second and third resonators are substantially adjacent and acoustically coupled together as are the third and fourth resonators. It will be appreciated by those skilled in the art that coupling, either electrical or acoustical is typically undesirable between nonadjacent resonators.
As shown in FIGS. 1 and 2A a wirebond pad 65 is coupled to electrode 25 by a narrow conductor 70. Wirebond pad 65 is connected by a fine bonding wire 75 to an external wirebond pad 80. This wirebond pad 80 may be used as the filter input and is connected to external circuitry as necessary to utilize the crystal filter.
In a like manner, wirebond pad 85 is electrically coupled to resonator 50 by a narrow conductor 90. Wirebond pad 85 is normally coupled by a fine bonding wire 95 to an external wirebond pad 100 which may be used as the filter output and is connected to other circuitry as necessary to utilize the filter. Similar narrow conductors 105 and 110 are shown clearly in FIG. 2A and are utilized to couple electrodes 30 and 35 to wirebond pads 115 and 120 respectively which are in turn coupled through wirebonds (not shown) to a circuit ground.
Turning now to FIG. 2B viewed in conjunction with FIG. 1, a pedestal mounting pad 130 is attached to a conductive mounting pedestal 135. As shown more clearly in FIG. 2B, pedestal mounting pad 130 is electrically coupled to each of electrodes 45, 50, 55 and 60 by narrow conductors 145, 150, 155 and 160 respectively. Pedestal 135 is in turn coupled electrically to electrical circuit ground in order to form a complete functioning crystal filter.
A schematic model of the resulting filter is shown in FIG. 3 wherein for simplicity the input terminal 200 is taken to be wirebond pad 65 and output terminal 205 is taken to be wirebond pad 85. In this simplified model the resonator formed by electrode pair 25 and 45 is modeled as a capacitor 210 in parallel with a series L-C circuit shown as capacitor 215 and inductor 220. The resonator formed by electrode pair 30 and 50 is modeled as capacitor 225 in parallel with the series L-C combination of capacitor 230 and inductor 235. Inductor 240 represents the inductance of the wirebond wire (not shown) coupling electrode 30 to ground. The resonator formed by electrode pair 35 and 55 is modeled by capacitor 245 in parallel with the series L-C circuit of capacitor 250 and inductor 255. Inductor 260 represents the inductance the bonding wire (not shown) coupling resonator electrode 35 to ground. The resonator formed by electrode pair 40 and 60 is modeled by capacitor 265 in parallel with the series L-C combination of capacitor 270 and inductor 275. For simplicity in the circuit, the internal resonator coupling has been represented by coupling co-efficients K1, K2, and K3. It will be evident to one skilled in the art that this acoustical coupling may be modeled schematically in many other ways, as for example by capacitive or magnetic coupling.
For the circuit of FIG. 3, capacitors 210, 225, 245, and 265 represent the parallel-plate capacitance of each individual resonator. This capacitance is formed by the sandwich-like structure of the resonator's upper and lower electrodes with crystal 10 serving as the dielectric. Capacitors 215, 230, 250, and 270 represent the motional capacitance of each respective resonator as is well known in the art. Inductors 220, 235, 255 and 275 represent the motional inductance of each individual resonator respectively as is well known in the art.
Inductor 280 represents the inductance of the mounting pedestal itself. The pedestal may be on the order of 0.04 to 0.06 millimeters in height and may have an inductance on the a nanohenries. Each of the resonators of the circuit of FIG. 3 is electrically coupled to node 290. This node represents the pedestal mounting pad 130. This node would ideally be coupled directly to ground, however, pedestal inductance 280 separates this node from ground and causes a finite amount of undesired coupling between non-adjacent resonators and particularly from the input resonator to the output resonator.
A frequency response plot of attenuation in dB vs relative frequency is shown in FIG. 4 for the circuit of FIG. 3. Curve 300 of that graph represents the frequency response of the circuit of FIG. 3. One skilled in the art will recognize that the crystal filter will have, for example, a small amount of insertion loss as a result of the finite Q of each of the individual resonators making up the filter. For sake of simplicy of the model, these resistive effects have not been taken into consideration. Therefore, although the plot of FIG. 4 (and subsequently FIG. 8) does not reflect the insertion loss, slight bandwidth change, etc, which will result from the finite Q of the crystal's resonators, it provides a sound basis for comparison for purposes of the present discussion.
In FIG. 4 curve 300 is seen to exhibit a pair of attenuation poles evidenced by dips in curve 300 at points generating designated 310 and 320. The exact location of these dips is somewhat difficult to control due to variations in processing, etc. These are undesirable attenuation poles and severely limit the ultimate selectivity (far-out stop band rejection) on both the high side and the low side of the crystal filter's response. In addition, due to the unpredictable nature of exactly where in frequency these poles will actually occur, they can and frequently do have the detrimental effect of altering the bandwidth of the filter response. These undesired poles are the direct result of parasitic pedestal inductance 280 which electrically couples non-adjacent resonators. The need to eliminate these undesired filter responses will be readily apparent to one skilled in the art.